This invention relates broadly to the field of quality control of manufactured parts. The typical quality control program involves some kind of comparison of the parts as made with the design specifications for the part. One way this has been done in the past was to build a checking fixture that mounted a part to be checked in an accurate, repeatable fashion on a base plate. The base plate carries a variety of mechanical devices accurately manufactured and located to correspond to the part as designed (the part as designed will be referred to herein as the perfect part or the nominal part). Thus, mechanical devices such as flush rails, feeler rails, templates and SPC (statistical process control) datum checks are built into the fixture to, in effect, simulate the perfect part. Parts to be checked are mounted in the fixture and compared with these mechanical simulations of the perfect part.
Flush rails are walls having an edge parallel to an edge of the nominal part. A straight edge held against the flush rail should also just be in contact with the edge of the part. Feeler rails are walls whose faces are located parallel to and a specified distance from the nominal part. A feeler gauge having a thickness equal to the specified distance is placed between the rail and part to check for discrepancies from the design. Templates are hinged blocks that fold down over the top of a mounted part. The contour of the template matches that of a perfect part. Again, a feeler gauge is placed between the template and part to test the accuracy of the part being checked.
SPC datum locators are mounting brackets for transducers or other distance measuring devices. Each SPC point has a bushing or other feature that is located normal to the nominal part and at a fixed, known distance from it. Typically, that distance has been chosen to be 31 mm. The transducer is calibrated to read variations from this known distance when the transducer is properly mounted relative to the bushing. Thus, to obtain correct results, the fixture has to be built such that the bushing (or other feature) is precisely normal to and, typically, 31 mm from the nominal part.
It can be seen that building these mechanical devices into a checking fixture requires slow, painstaking effort often involving repeated cycles of fabrication, checking, correcting and rechecking. For example, consider an SPC datum locator that is mounted to a base plate by means of holes in the base. Once the holes are drilled they are checked for accuracy on a coordinate measuring machine (CMM). If a hole is not located within a few ten-thousandths of an inch of the design, it must be plugged (by pounding a chunk of metal into the hole in a press fit), welded, filed and polished so that the holes can be redrilled, hopefully in the right place. Naturally all this effort risks adversely affecting other parts of the fixture already in place. Also quite naturally this effort leads to high costs for a checking fixture which, it will be noted, may be suitable for use only with one, unique part.
The present invention overcomes the difficulties of simulating the nominal part with physical, mechanical components mounted on a base. It does so by dispensing with devices dependent on feeler gauges, using only distance measuring devices whose holders need not be located with absolute precision.